The present invention relates to gate valves and, more specifically, it relates to improvements to a gate valve having a secondary seal.
As discussed in U.S. Pat. No. 4,377,273, one problem frequently encountered in the case of gate valves is the lack of formation of adequate seals between the gate or gates and the seat or seats. In turn, if a seal is inadequate, the gate valves are often prone to leak through the seal. In U.S. Pat. No. 4,377,273, an invention is disclosed which seeks to provide an adequate seal for gate valves in the form of a positive bidirectional cam actuated and deactuated resilient secondary fluid seal.
While the invention described in U.S. Pat. No. 4,377,273 is believed to be advantageous for many applications, it has been found that there are applications in which other secondary seals may be provided. In particular, in high pressure situations wherein the formation of the secondary seal causes a significant pressure differential on the two sides of the gate and wherein the secondary seal is located on the downstream side, it has been found that for some applications, the drag on the secondary seal caused by movement of the gate to open when the secondary seal is fully effected may exaggerate the wear of the secondary seal. The additional drag on the gate also increases the effort required to open the valve. It is desirable in those instances to be able to deenergize or release the secondary seal prior to substantial linear movement of the gate across the seal.
In this regard, it is not necessary to totally withdraw the seal from the gate, but rather it is desirable to relieve the full pressure of the seal against the gate, or "deenergize" the seal, such that the pressure of the fluid moving through the valve will flow between the seal and the gate to preclude wear.
Additionally, in some high pressure situations wherein the secondary seal is fully effected or energized with no pressure in the valve body, and pressure is thereafter applied from the upstream side, there is no way for the fluid to enter into the body without leaking past the energized upstream secondary seal. Since the upstream secondary seal is mechanically energized, it is maintained until the pressure load of the fluid overcomes the mechanical load. In some instances, the pressure of the fluid is great enough to blow past the seal once it overcomes the mechanical energy of the seal, thereby damaging the resilient sealing ring contained in the seal.
It is therefore desirable to provide a pressure relief means for a secondary seal subjected to such pressurized fluid under the above conditions, in which the fluid will force the seal away from the gate and allow movement of the fluid past the seal without damaging the resilient seal element when the pressure of the fluid reaches a selected value.